The Chapter opens again with the usual discussion of the problem with how Korsgaard and O'Neill are said to take the Formula of Humanity. However, when he reaches the account of "rational consent" Parfit this time refers to a point not included in either of the two previous treatments. This concerns a problem with thinking that referring to shared ends will suffice to make rational consent sufficiently inclusive. Here Parfit points out that whilst there can be an agreement on ends this does not necessarily translate into an agreement concerning the appropriate means for attaining these ends.
Subsequently a reference to Rawls appears which cites him as interpreting the consent principle as meaning that we consent rationally to some act if and only if or "just when" we could will it to be true that the agent's maxim is a universal law. This requires invocation of the Formula of Universal Law. The reason why Rawls refers to this is due to Kant's general claim that all the formulas mean the same thing or are "statements of the same law". Rawls assumes that this means that Kant cannot have added something to the content of the law when he states one formula rather than another. But Parfit does not accept this view and assumes, rather, that there is something in the Formula of Humanity that is not included in the Formula of Universal Law. In making this assumption Parfit is following the precedent of, for example, Allen Wood, who, likewise, assumes that the Formula of Humanity has importantly different implications than the Formula of Universal Law. This point is not a small one since the discussion of the relationship between formulas of the categorical imperative has been a major source of disputes between interpreters of Kant. Unfortunately, whilst Rawls' reason for assuming that the Formula of Humanity is not significantly different to that of Universal Law, seems rather simplistic, it is hardly helpful of Parfit to simply stipulate that he does not accept this view without arguing on both philosophical and textual grounds for taking the formulas to be different. Such an argument does require, further, some discussion of what Kant means by claiming that there is no new "content" added in any of the formulas.
Parfit also adds in this draft a claim to the effect that rational consent has to be "informed" consent, a proviso not previously made clear. The Consent Principle now becomes:
"It is wrong to treat people in any way to which they could not rationally consent in the act-affecting sense, if these people knew the relevant facts, and we gave them the power to choose how we treat them."
The notion of "treating" is also to be understood in a sufficiently broad way so as to include, for example, breaking promises to the dead. The notion that is really involved for the consent to which the principle refers is "sufficient reason" in the sense that we consent to that which we do not have sufficient reason to refuse to consent. This implies a shorter formula of the Consent Principle which Parfit also gives but which I'll leave aside here.
There are clear constraints upon the Consent Principle since it should be both plausible in itself and have plausible implications. These constraints are clear concessions on Parfit's part to intuitive or common-sense conceptions of what morality requires and forbids and echo the concession made in Climbing the Mountain to the point that beneficence that makes oneself in need of beneficence is misdirected. These points are subsequently mobilised by Parfit to suggest that the Consent Principle cannot be integrated with either egoistic or "subjective" (desire-based) views of the good.
The appeal to "sufficient reason" is later finessed by Parfit into a view about "facts" that pertain in situations such that they are what make the beliefs concerning the rationality of consent plausible or otherwise. However, Parfit's subsequent consideration of examples leads to the same view as in Climbing the Mountain, namely that the Consent Principle may be too demanding (and hence fail to meet the constraint of having plausible implications). Further, it is not alone sufficient to describe what it is for something to be morally right since it turns out that it is possible to rationally consent (on Parfit's view) to things that are morally wrong. And, as in Climbing the Mountain, this turns out to be the ground on which Parfit moves to the discussion in the Formula of Humanity concerning treating others in such a way that they are not "merely used as means".